Polymer-coated particles composed of various kinds of particles and polymers have been widely used in the industrial field. In recent years, such polymer particles are increasingly important in the fields of medicine and fundamental biology. For example, there is growing interest in applications to affinity chromatography carriers, medical diagnosis, drug delivery system (DDS), and drug development. As an example of the application to drug development, biological molecules such as specific proteins are trapped by biologically active substances called ligands which have been fixed to various particles (carriers), and then the biological molecules are separated and refined.
A particle used as a carrier is required to satisfy the following conditions: (1) the particle has a functional group capable of fixing a large amount of ligand or spacer under a mild condition; (2) the carrier does not nonspecifically adsorb biological substances such as protein; (3) the particle has a mechanical strength suitable for the intended use. However, there is no carrier which satisfies these conditions.
The performance of a carrier largely depends on the amount of ligand to be fixed, so that a carrier is required to fix as much ligand as possible. The larger the amount of ligand is fixed, the larger the total amount of protein is trapped. In particular, when the target protein is a minor component among the proteins to be trapped by the ligand, the target protein may be not detected if the fixed amount is too small. Accordingly, particles capable of fixing a large amount of ligand have been demanded.
It is also very important to inhibit nonspecific adsorption of components other than the protein to be trapped. Among known carriers, inorganic carriers composed of silica gel particles are expected to be suitable for separation because they are porous and have high physical strength. However, silica gel particles have a disadvantage of causing high nonspecific adsorption, so that they are scarcely used for practical purposes. As synthetic polymer carriers, particles composed of a polyacrylamide gel (trade name: Bio-GelP, manufactured by Bio-Rad Laboratories, Inc.), polystyrene, or an ethylene-maleic anhydride copolymer have been developed. However, these polymers also have a disadvantage of being prone to nonspecific adsorption of biological substances.
As described above, when particles are used as medical carriers, their nonspecific adsorption often present problems. In order to avoid the problems, various techniques have been studied. An example is a blocking method wherein a target biologically active substance is attached to the particle surface, and then a harmless protein such as bovine serum albumin (BSA) is attached to the rest portion of the particle surface. However, the effect of the method is not sufficient. In another method, DNA which specifically binds to a specific protein is attached to particles having a surface composed of an epoxy group-containing resin, and the particles are used for refinement of the protein (for example, Patent Document 1). In the method, epoxy groups are introduced to the particle surface by glycidyl methacrylate or the like because it less exhibits nonspecific adsorption to proteins. However, direct binding between epoxy groups and a biologically active substance such as DNA requires significantly severe conditions. Therefore, the method is improper when the biologically active substance to be fixed is unstable toward alkalis and at high temperatures, because the biologically active substance may be deteriorated during the fixing process. As exemplified by the examples, when particles having a surface composed of an epoxy group-containing resin are synthesized by polymerization, the control of the particle size is difficult as with the case described in Patent Document 2. Particles having a core composed of a polymer compound cannot have higher specific gravity than those composed of an inorganic material, so that they are difficult to be separated and collected.
Another method for reducing nonspecific adsorption to particles is the synthesis of polymer particles with minimal nonspecific adsorption through emulsion polymerization or the like. For example, Patent Document 2 describes a method for producing copolymer particles through emulsion polymerization or suspension polymerization of an ethylenically unsaturated polymerizable monomer having a reactive group reacting with a biologically active substance, an ethylenically unsaturated polymerizable monomer having a polyoxyalkylene side chain, and an ethylenically unsaturated polymerizable vinyl aromatic monomer conferring hydrophobicity. However, with these methods, it is difficult to control the diameter of the resultant particles. In general, particles having a relatively uniform particle size are produced by emulsion polymerization, but the particle size is limited to the order of submicron. On the other hand, the particles produced by suspension polymerization have a large particle size distribution, so that they must be subjected to classification when used as, for example, a column filler. However, it is difficult to minutely classify the polymer particles without a special apparatus. In addition, the size of the obtained particles range from several tens to several hundreds of nanometers, and it is difficult to synthesize particles having a smaller diameter. Furthermore, particles obtained by polymerization are polymers, so that the strength of the carrier is inevitably limited. For applications where high strength is required, it is inevitably necessary to increase the particle strength by, for example, increasing the ratio of the ethylenically unsaturated polymerizable vinyl aromatic monomer in the polymer. However, this is disadvantages for nonspecific adsorption.
At present, most frequently used carriers are porous agarose particles (trade name: Sepharose) and dextran particles (trade name: Sephadex) accompanied by various functional groups. Agarose gel is said to be suitable for use with high molecular weight biological substances because it can remove higher molecular weight impurities in comparison with dextran or acrylamide. However, these carriers are less resistant to pressure because of their low physical strength, and have many other problems such as unavoidable inclusion of impurities caused by adsorption or retention of impurities in the network structure of the carrier.    [Patent Document 1] Japanese Patent No. 2753762    [Patent Document 2] Japanese Patent No. 3215455